qapi/qmp-dispatch: fix return value in do_qmp_dispatch

There are no harm but just looks weird to return bool in
pointer-returning function. Introduced in 69240fe62d1 with the whole
failure-checking "if" chunk.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190325154748.66381-1-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qapi/qmp-dispatch.c

@@ -111,7 +111,7 @@ static QObject *do_qmp_dispatch(QmpCommandList *cmds, QObject *request,
     if (oob && !(cmd->options & QCO_ALLOW_OOB)) {
         error_setg(errp, "The command %s does not support OOB",
                    command);
-        return false;
+        return NULL;
     }
 
     if (runstate_check(RUN_STATE_PRECONFIG) &&